Carefully remove the screws and unplug the bed from the motherboard. The bed will not fit through the retainer. If you do not feel comfortable disassembling the printer that far, just snip one of the sides of the retainer. Use some sandpaper and score around the heat plate for better glue adhesion.
Tip: After scoring the bottom-side of the bed, use some high-temp RTV to protect the wires. I had one break off and require some higher temp solder to weld back in place.
Dab some glue on the retainer and use some binder clips or bulldog clamps to hold in place while the glue sets.
Optional, place a small bead of glue on the front-side.
I recommend complimenting the retainer with a set of Magnetic Bed Fasteners.
Also, not all glass beds purchased are created equal. I had one bed that was slightly smaller in diameter which caused the clips to be loose. I used a hot air gun to softened the clips and shaped them on the bed by holding them in place while they cool (be careful, don't burn yourself!).